Despite the aura of omnipotence most empires project, a look at their history should remind us that they are fragile organisms. So delicate is their ecology of power that, when things start to go truly bad, empires regularly unravel with unholy speed: just a year for Portugal, two years for the Soviet Union, eight years for France, 11 years for the Ottomans, 17 years for Great Britain, and, in all likelihood, 22 years for the United States, counting from the crucial year 2003.